From the sound of the titles alone, the latter builds on the former, not supercedes it. I'm guessing My System will remain an excellent read ;)
Nimzowitsch - My system

Silman points out that people who are just learning might find Watson's book confusing in that Watson's main point is that all the old rules have problems with them. I think it's probably better to learn the rules first before worrying about the exceptions. There is a new translation of My System out now that is in algebraic notation. Easier books about strategy are Simple Chess by Michael Stean and Simple Chess by John Emms. Neil McDonald's Chess Secrets: The Giants of Strategy is another fairly easy book that touches on a lot of Nimzowitsch's themes: seventh rank, pawn breaks, restraint, blockade and prophylaxis.

Is Chess Praxis still in print? I lost my copy awhile back.
Yes...amazon.com shows several for sale.

I just bought (I hope) the newest version of My System (version 2007, ISBN 9789197600538).
Looking forward to read it after these comments!
I have My System....
but there is another book out there,
Secrets of Modern Chess Strategy: Advances sinces Nimzowitch.
Does the latter supercede the former...?
Or are they complimentary?